(FORTUNE Magazine) – If there's one thing that has kept this bull market from swelling into a ready-to-pop speculative bubble, it is the decade's astonishing growth in corporate profits. Even if it's really mutual fund inflows or mass hysteria that's been driving stock prices ever higher, profits have risen quickly enough to keep price/earnings ratios more or less within reason.

So when the great profit boom that started in 1992 shows signs of faltering, it's a big deal--and skyrocketing price/earnings ratios would be just one of the consequences. Past profit slowdowns have often signaled the onset of a recession. Further, an end to the 1990s' remarkable string of profit increases would challenge some popular notions about a transformation of the U.S. economy into a leaner, more efficient machine.

Is there in fact evidence of a profit slowdown? Sort of. The Blue Chip consensus of economic forecasters puts profit growth at 4.7% in 1998--after gains of at least twice that in each of the past five years. But economists have been assuming for a few years now that profit growth had to cool down. So far they've been wrong, although this year's lineup of reasons for a slowdown is pretty compelling: shrinking exports due to Asia's troubles (witness January's trade deficit, the biggest since 1987); rising labor costs; and massive expenses to fix year-2000 computer problems.

Even more compelling, if seldom used in the past as economic indicators, are some figures from I/B/E/S, which compiles the earnings forecasts that securities analysts make for individual companies. The analysts are suggesting that profit growth is about to grind to a halt, at least temporarily. They are, as is their custom, still bullish about 1998, forecasting 9.7% earnings growth for the S&P 500. But that forecast is down from 14.3% at the beginning of the year. The profit-growth forecast for the first quarter has been downgraded even more sharply, from 9.6% at the beginning of the year to 1.1% as of March 20--which would be the smallest year-over-year profit increase since 1991. The analysts' short-term predictions have become even more pessimistic than the marketwide forecasts I/B/E/S gathers from Wall Street strategists and economists--which never happens. The significance of it all isn't clear, but analysts' behavior does bear an alarming resemblance to that of, say, barnyard animals before an earthquake. "It's a little worrying, these dizzying drops," says I/B/E/S senior vice president Ed Keon.

So far the only solid facts about the 1990s profit boom, however, are the historical ones, and they just show its strength. Measured in dollars, corporate earnings doubled between 1992 and 1997. On the less forgiving scale of profits as a percentage of GDP, profits are at levels not seen since the 1960s. Return on equity is the highest it's ever been.

Presumably this can't go on forever: At some point in every business cycle, profits do start to decline. Early in an economic expansion, underutilized factories and unemployed workers make it easy for companies to produce more while keeping costs down. Later in the cycle, production capacity becomes scarce, the labor market tightens, and profit margins shrink. To some, that's what has been at work in the 1990s. "There's no big surprise here--you hold wages down, and profits are going to go up," says Dean Baker, an economist with the left-leaning Economic Policy Institute. "It's hard to see profits rising further."

But there are tantalizing hints that the 1990s profit boom might be different. For one thing, this business cycle has been on an upswing for an abnormally long time without showing any of the usual signs of strain (e.g., inflation). There are also new technologies and management techniques at work that ought to be making U.S. companies markedly more productive. This "Brave New World" thinking has gained a lot of adherents on Wall Street, and it's as unproven as it is attractive.

The best empirical support for the Brave New World thesis would be if corporate profits were to keep going up and up. The current profit boom has been so strong and prolonged that even some academic economists, who jump to conclusions far less quickly than their Wall Street brethren, are looking into it. But don't expect blinding revelations soon. Says MIT economist James Poterba: "It hasn't lasted long enough to look like we've moved into a new profit regime."

Of course, even if predictions of ever higher profitability turn out to be true, there will come a point where the increases will stop looking so impressive--and price/earnings ratios will settle down. As UBS Securities' chief economist Paul McCulley put it in a recent report, "Living in the Brave New World will not be nearly so much fun as getting there." Investors can only hope the ride's not over quite yet.